Turbine bucket for exhaust turbine superchargers



Alli 7 .1945- A, w.. MERR|cK 2,381,459

TURBINE BUCK-EIT FOR EXHAUST TURBINE SUPERCHARGERS Y Filed Dec. 1o, 1941INVENTOR Patented Aug. 7, 1945 v TURBINEBUCKET FOR EXHAUST TURBINESUPERCHARGERS Albert W. Merrick, Ossining, N. Y., assignor to AustenalLaboratories, Inc., New York, N. Y., a corporation of New YorkApplication December 10, 1 941, SerialNo. 422,421 (Cl. 253-77) 5 Claims.This invention relates, generally, to exhaust turbine superchargersforinternal combustion engines, and it has particular relation to animproved turbine bucket for superchargers of the class described.

'I'he invention may be embodied in a wide variety of forms of turbinebuckets for use with a wide variety of exhaust turbine superchargers.For purposes of illustration I shall refer more or less generally to adiagrammatically illustrated form of exhaust turbine supercharger and toan illustrative form-of turbine bucket, but it is to be understood thatthe invention is not limited to use with the particular form ofsupercharger selected for illustration, nor to embodiment in theparticular form of turbine bucket shown and described.

superchargers are required for high power output and are desirable foraircraft engines, for example, for take-olf power, and to compensate forthe rare atmosphere at high altitudes. 'I'hey are also desirableforautomotive engines at high speeds, and Yfor Diesel engines forincreased output.

'I'he rotors of these superchargers run at very high peripheralspeeds-at substantially the speed of a riie bullet-and the turbinebuckets operate within the path of the exhaust gases where thetemperatures are very high-of the order of from about 1400" F. to 1500F. The turbine buckets, therefore, are subject to very severeconditions, particularly in regard to temperature and the high stressesto which they are subjected as the res'ult of the action of centrifugalforce.

Heretofore, forged and machined buckets have usually been employed, andthese buckets have necessarily been formed of alloys which would permitthe forging and machining operations.

With the improvement of aircraft and their engines, the necessity forbetter turbine buckets for exhaust turbine superchargers and bettermaterials for such turbine buckets has become urgent.

The improvement of the present invention corn sists in forming thebucket by a casting process and of an alloy providing new andadvantageous results not equalled by the alloys previously used inmaking such bucket, and which alloy it would be hopeless to consider fora forged and machined bucket on account of the hardness and difficultyof machining and impracticability of forging the same.

More specifically, the improvement of the present invention consists inproviding a cast turbine bucket for exhaust turbine superchargers,

such bucket being formed of a cobalt-chromium alloy, and, moreparticularly, formed of a cobalt, chromium, molybdenum alloy With theconstituents combined and` proportioned in a. manner better to withstandthe severe conditions to which such buckets are subjected.

Further and 'more specific features and advantages of the invention willappear from the following detailed description taken in connection withthe accompanying drawing, in which:

Figure 1 is a diagram of one form of an exhaust turbine superchargerwith which the turbine' buckets of the present invention are adapted tobe used;

Figure 2 is a fragmentary view showing one illustrative manner in whichthe turbine buckets may be attached to the periphery of the 'rotor orturbine`whee1;

Figure 3 is a back view of one of the turbine buckets;

FigureA is a front view of the turbine bucket The exhaust turbinesupercharger comprises a combined turbine and compressor shaft I3 havingfixed thereon the rotor or turbine wheel I4 and an air impeller I5.'I'he turbine buckets I6 are fastened, as will hereinafter appear, tothe periphery of the rotor or turbine wheel I4 and operate Within thepath of the exhaust gases which serve as the motive uid for turning theturbine wheel and thereby the shaft I3 and air impeller I5. In aircraftengines, the exhaust gases will drive the turbine wheel at very highperipheral speed, commonly from seven to twelve times crankshaft speed,or at substantially the speed of a rifle bullet.

'I'he air impeller I5 operates Within the impeller housing I8 which hasan air compressor inlet I9 and an air discharge 20 to the carburetorinduction pipe 2 I. The air impeller I 5'forces air through thecarburetor shown diagrammatically at 22, and the explosive mixture isdelivered from the carburetor through the intake II into the cylindersof the engine, one of which cylinders is shown'. The carburetor may, ofcourse, be located before the supercharger instead of after the same, asshown in the drawing. The intake into the cylinder of the engine iscontrolled by the usual or any suitable intake valve 23, and

the exhaust from the cylinder of the engine is controlled by the usualor any suitable exhaust lvalve 24.

' preferably cast from an alloy whose essential or principal ingredientsare cobalt and chromium. In the broader aspects of the invention, thecobalt is present as the principal ingredient, and more specifically, inamount more than 50%, and the chromium is present to the extent of fromapproximately to approximately 40%.

For a full understanding of the various alloys from which I contemplate,within the broader aspects of the invention, casting or forming theturbine buckets I6, attention is directed to the alloys more fullydescribed in Charles H. Prange Reissue Patent No. 20,877, reissuedOctober 4, 1938; also to Charles H. Prange Patent No. 2,135,- 600,patented November 8, 1938, and to Charles H. Prange Patent No,2,180,549, patented November 21, 1939.

One preferred form of alloy from which highly satisfactory turbinebuckets have been made is substantially as follows:

Per cent Cobalt 63.0 Chromium 30.0 Molybdenum 6.0 Silicon 0.25 Manganese0.50 Carbon 0.25

In addition, there are likely to be small quantities of iron and nickel,but these are simply impurities and are not introduced purposely.

As a commercial specification, substantially the following range ofcompositions would cover this latter alloy as it could be produced on acommercial basis:

Percent Cobalt 60.0 to 65.0 Chromium 28.0 to 32.0 Molybdenum 5.0 to 7.0Silicon .10to 1.0 Manganese .20 to 1.0 Carbon .00to .50

The physical properties of the preferred form i of turbine bucket alloyabove set forth as exemplified by actual tensile tests are as follows:

Ultimate strength lbs. per sq. inch 110,000 114,000

Yield point 0- 71,000 Elongation per cent" 12.0 14.0 Reduction of areado 14.0 12.0

The above tests are at room temperature. At

These figures show the excellent properties of the composition in thecast condition, the one set at room temperature and the other at 1500 F.This is illustrative of the excellence of the alloy for the hightemperature conditions to which turbine buckets for exhaust turbinesuperchargers are subjected.

The mold and method for its production disclosed in Arthur B, Ray PatentNo. 2,027,932, patented January 14, 1936, as well as the castingprocedure disclosed in Charles H. Prange reissue patent Reissue No.20,877, reissued October 4, 1938, and the casting investment materialand process disclosed in Charles H. Prange Patent No. 2,180,549,patented November'21, 1939, are highly suitable and advantageous in thecasting of the turbine buckets of the present invention, and referenceis hereby incorporated herein for the further details 'of these castinginvestment materials and processes as casting investment materials andprocesses suitable for casting the turbinel buckets of the presentinvention.

The founding apparatus and method disclosed in the present applicantsprior Patent No. 2,125,- 080, patented July 26, 1938, are also highlysuitable and advantageous in the casting of the turbine buckets of thepresent invention, and reference to that patent is hereby incorporatedherein for the further details of the founding apparatus and method asthe same may be used in connection with the present invention.

I find that the characteristics of the alloy, when applied to turbinebuckets for exhaust 'turbine superchargers for internal combustionengines, impart greatly desired properties not provided in the turbinebuckets of the prior art.

Most alloys suffer a very marked reduction in strength at elevatedtemperatures, but a turbine bucket made from the alloy described hereinretains a proportionately greater strength at such temperatures.

A further aspect is the matter of creep" strength. `This refers to thegradual stretching of a metal under stress at elevated temperatureswhich ultimately results in failure. It differs from the matter of hotstrength" in that the time element is involved. Turbine buckets formedof an alloy as herein disclosed have excellent resistance to creen Thisshould not be confused with red hardness which is not the same as hotstrength.

Turbine buckets formed of an alloy as herein disclosed also have greatresistance to oxidation and corrosion. The exhaust gases of an airplaneengine are of high temperature and contain corrosive products such assulphur compounds and possibly also anti-knock compounds. At any rate,turbine buckets of the class described are subjected to severe oxidizingand corroding conditions, and it has been found that buckets formed ofan alloy as herein disclosed stand up exceln lently in this type ofservice. On tests, they have shown no deterioration other than a slightsurface discoloration, whereas buckets made out of other alloysforinstance, nickel alloyshave shown definite deterioration.

As a further aspect, there is the matter of resistance to erosion by hotgases. Hot gases and vaporsas for instance, steam, have a tendency towear away metal by their mechanical action. In other words, they causeerosion. Where this is combined with the effect of corrosion andoxidation, as in turbine buckets of the class described, the effect maybe quite severe. Buckets formed of an alloy as herein disclosed haveexcellent resistance to4 erosive influences.

There are also other important aspects-for instance, the matter ofslight shrinkage upon solidifying-Which enter into the making of soundturbine bucket castings.

Moreover, the alloy bucket as herein disclosed h'asa very high degree ofresistance to repeated ance to fatigue breakage is related to thefavorable damping characteristics of the alloy in its cast form. In thisconnection it will be noted that, in general, cast metals have a greaterdamping effect than forgedor machined metals.

Referring to the bucket as a oast product, it is important to note thatcast buckets which have been made h'ave proved to be sound, in general,whereas the forged buckets have given a great deal of trouble due tointernal defects, such as forging cracks which are hard to discover byX-ray. The Y alloys which are suitable for turbine buckets on account oftheir strength and hardness at high' temperatures are generally hard tohandle by forging. Consequently, there is a great likelihood of cracksin forged and machined buckets.

Buckets made in accordance with the present invention have been testedby X-ray and have been found to be uniformly sound and reliable. Forgedbuckets, on the oth'er hand, sometimes have concealed defects which aredifllcult to vdiscover, even by X-ray, such as internal cracks resultingfrom the forging operations. In an article, such as a turbine bucket forexhaust turbine superchargers, this, of course, is a very seriousobjection. As a result of the casting method, fthe accuracy of thebucket is suchI that machining and grindying are reduced to an extremelysmall amount.

With the low carbon content as set forth in connection with thepreferred forms of alloy, tough cast buckets are produced. With this lowcarbon content the molybdenum content is believed to makev up thenecessary strength and stiffness. Toughness and cold ductility arehighly advantageous properties in buckets of th'e class described. y

With the low carbon content as set forth-in connection with thepreferred forms of alloy, the resulting buckets appear to be better onaccount of there being less likelihood of internal structural changesunder the conditions of service to which such buckets are put.

I am unable to state with certanty all considerations in connection withthe present invention, and therefore I reserve the right to supplementand correct any considerations herein set forth. For example, it isconceivable that with higher carbon content, carbide preci'plation mayoccur through the action of high temperature and time which would resultin embrittlement of the bucket.

As will appear from Charles H.y Prange Patent No. 2,135,600, molybdenummay be substituted for carbon, and a` substantially carbonless alloy ofhigh strength and corrosion resistance may be made by increasing thepercentage of molybdenum. As indicated in the last mentioned patent,molybdenum, to excess, will cause both brittleness and high meltingpoints.

There are other considerations in connection with carbon about which Iam not fully aware. For instance, it is conceivable that a compositionhaving a substantially zero carbon content might not be stable whenexposed to exhaust gases which contain carbon monoxide and carbondioxide at highl temperatures. In' short, a carburizing effect mayoccur, so that th'e carbonless alloy would take up carbon gradually andbecome brittle. It is possible in Athis connection that the preferredforms of alloy-and particularly the alloy containing about .25% carbonisan exceptionally favorable one forthe purposes of the present inventionin that having some carbon in it, it would be less likely to take upmore carbon and thereby become brittle.

It is to be understood that tungsten may operate in a, similar role tomolybdenum as replacing carbon, increasing amounts adding strength andstiffness, and, of course, there is the limitation as expressed in thepatents h'ereinbefore identified that excesses of tungsten will producebrittleness and diillcult melting.

As between molybdenum and tungsten, it is felt that molybdenum ispreferable. With it, it is possible to retain a greater degree oftoughness or ductility while at the same 'time attaining strength andstiffness. Further-incre, it is found that the molybdenum'alloy hassomewhat great corrosion resistance than th'e tungsten alloy.

'Ihe resulting buckets have not only accuracy and the other propertiesherein set forth, but they are smooth of surface. This results mainlyfrom the character of the alloy and the method of spraying on a thincoat of investment over the Wax pattern before investing it. Theinvestment herein referred to is inert so that no carburization ordecarburization or sulphidization occurs.

Referring again to the drawing, the cast buckets I8 selected forillustration have concave fron-t or leading surfaces 28 arcuate in,transverse section as shown in Figure 5 and extending radially from th'eperiphery of the rotor or turbine Wheel I4 when the buckets are appliedth'ereto. The back surface 29 of the bucket follows generally thecontour of the concave fron't surface 2,3. The inner end of the buckethas an enlarged flattened neck or tongue 30 adapted to enter atransverse slot 3l in the periphery ofthe turbine wheel I4. Extendingtransversely along th'e inner end of the neck or tongue 30 is anintegral rounded head or enlargement 32 which enters a correspondingenlargement 33 at the inner end of the slot 3| for fastening the bladeor bucket in place on the periphery of the wh'eel I4. Alternate bucketsI6 preferably have long and short necks 3|) as shown in Fig-ure 2, inorder to avoid weakening of the wheel Il, a continuous circle of thebuckets I6 being mounted around the periphery of the wheel as shown.

The outer ends of the buckets I 6 have generally rectangular end walls3l which cooperate, as shown in Figure 2, when the buckets are in placeon the wheel. Transverse ribs 35, one integral with the back of eachbucket, seat in and interlock with transverse grooves 36, one in thefront of each bucket when the buckets are in place in the periphery ofthe wheel.

The circle shown in dotted lines at 38 in Figure 3 indicates theposition of small circular projections which are the remains of thesprues where they have been cut off. These small circular projectionsare preferably ground oif or removed, and, therefore, are not shown inFigure 2.

The embodiment of the invention shown in the drawing is for illustrativepurposes only, and it is to be expressly understood that said drawingand the accompanying specification are not to be construed as adefinition of the limits or scope of the invention, reference being hadto Attempts have been made to make turbine buckets bypowder-metallurgy-that is. by pressing and sintering. So far, theseattempts have not been successful because they have not been able to getthe required properties. 'I'he alloy herein disclosed in cast form isstrongerv and does not have the minute porosity found in articles moldedfrom powder. One disadvantage of this porosity is a decreased thermalconductivity which may be important in an air-cooled bucketthat is, onehaving an internal cooling passage.

I also contemplate,wlthin the scope oi' the appended claims, casting theWhole turbine wheel in one piece, that is, casting the wheel and thebucket as an integral or unitary construction.

I claim:

1. As a new article of manufacture, a turbine bucket for a turbine wheelin which the bucket is directly exposed to a high temperature highvelocity stream of combustion gases for driving the wheel at highperipheral speed, said bucket being cast of a cobalt-chromium alloycontaining cobalt 50% to 70%, chromium 20% to 40%,

molybdenum 3% to 7%, and carbon up to 0.5%, said alloy bucket beingpractically'incapable of being machined and Worked and having hightensile strength and high resistance to corrosion and erosion by thecombustion gases at temperatures on the order of 1500 F.

2. As a new article of manufacture, a turbine bucket `for a turbinewheel in which the bucket is directly exposed to a high temperature highvelocity stream of combustion gases for driving the wheel at highperipheral speed, said bucket being cast of a cobalt-chromium alloycontaining cobalt 50% to 70%, chromium 20% to 40%, metal from the groupconsisting of molybdenum and tungsten 3% to 7%, and carbon up to 0.5%,said alloy bucket being practically incapable ot being machined andworked and having high tensile strength and high resistance to corrosionand erosion by the combustion gases at temperatures on the order of 1500F.

3. As a new article of manufacture, a turbine bucket for a turbine wheelin which the bucket is directly exposed to a high temperature highvelocity stream of combustion gases for driving the wheel at highperipheral speed, said bucket being composed of an alloy containingcobalt to 70%, chromium 23% to 32%, molybdenum up to 7%, and carbon upto 0.5%, said alloy bucket being resistant to the high temperatures andaccompanying high stresses at the periphery of the turbine wheel.

4. As a new article of manufacture, a turbine bucket for a turbine wheelin which the bucket is directly exposed to a high temperature highvelocity stream of combustion gases for driving the wheel at highperipheral speed, said bucket being cast of an alloy containing cobalt60% to '70%, chromium 23% to 32%, molybdenum up to 7%, and carbon up to0.5%, said alloy bucket being resistant to the high temperatures andaccompanying high stresses at the periphery of the turbine wheel.

5. A turbine bucket for a turbine wheel in which the bucket is directlyexposed to a high temperature high velocity stream of combustion gasesfor'driving the wheel at high peripheral speed, said bucket beingcomposed of an alloy containing cobalt approximately chromiumapproximately 27.5%, molybdenum 5% to 6%, and carbon approximately0.25%, said alloy bucket being resistant to the high temperatures andaccompanying high stresses at the periphery of the turbine wheel.

` ALBERT W. MERRICK.

